This invention pertains to telephone switching systems, in general, and to apparatus for interfacing digital transmission lines and digital telephone switching systems in particular.
Where a digital transmission line is used to interface a digital switching network of a Central Office with remote equipment such as a remote line switch, a channel bank or another digital office, it is known to provide a carrier terminal unit or span interface to provide terminating functions for the transmission line such as described in copending application Ser. No. 937,456 filed Aug. 28, 1978 by F. Fellinger et al and assigned to the same assignee as the present application. As pointed out in the above-referenced application, the primary digital carrier, DS1, employed in North America, utilizes 24 pcm channels at a 1.544 Mb/s bit rate with D2/D3 signalling format. In a recently developed family of time-division multiplexed switching systems developed by ITT North Electric Company previously identified as "DSS" and now referred to as System 1210, it was found to be particularly advantageous to switch voice paths at a 2.048 Mb/s bit rate. Various aspects of the "DSS" or now "System 1210" family of systems have been described by N. J. Skaperda in "Generic Digital Switching Systems", International Switching Symposium, Vol. 1, October 1976; by C. G. Svala in "DSS-1, A Digital Local Switching System with Remote Line Switches", Proceedings of the National Telecommunications Conference, p 3915-1, 1977; and by F. Fellinger in "Modular Digital Switching Network", International Communications Conference, June 4-7, 1978.
The carrier terminal unit described in the above-referenced application is particularly suited for interfacing between a remote line switch and a central office. Where a local line switch is employed, the complex synchronization equipment needed for the operation of the remote line switch is not required. More specifically, because of the proximity of the local line switch to the digital network, the network clock can be distributed directly to the line switch to provide timing, i.e., it is not necessary to derive clock signals from the incoming pcm data stream. Additionally, framing circuits, the elastic store arrangement, unipolar/bipolar and bipolar/unipolar conversions of the data, as provided in the carrier terminal unit, are not required. However, it is necessary to provide an interface between the local line switch and the network which provides rate conversion, bit alignment and signalling format conversion functions.